1. Field of the Invention
The present invention generally relates to a cellular wireless communication system, and more particularly to a method and system for transmitting/receiving a common control channel for the transmission of downlink system information, in particular, a broadcast channel.
2. Description of the Related Art
In recent years, Orthogonal Frequency Division Multiplexing (OFDM) technology has been widely employed in broadcasting and mobile communication systems. OFDM technology is advantageous in that it removes interference between multi-path signal components existing in a radio communication channel, ensures orthogonality between multiple access users, and enables frequency resources to be efficiently used. On account of this, OFDM technology is useful for high-speed data transmission and wideband systems, as compared with Direct Sequence Code Division Multiple Access (DS-CDMA) technology, such as Wideband Code Division Multiple Access (WCDMA), CDMA2000 or the like.
FIG. 1 shows a conventional OFDM signal structure in the time-frequency domain.
In FIG. 1, one OFDM symbol 100 occupies N subcarriers 102 in the frequency domain. Individual modulation symbols 104 for transmission information are carried and simultaneously transmitted in parallel by the respective subcarriers 102. A modulation symbol 104 transmitted by each subcarrier 102 is referred to as a subcarrier symbol. The OFDM technology as described above is a type of multicarrier transmission technology in which data or control channel information to be transmitted can be separately carried and transmitted in parallel by several subcarriers. In FIG. 1, reference numerals “106” and “108” designate ith and (i+1)th OFDM symbol intervals, respectively. Each physical channel in an OFDM-based communication system includes one or more subcarrier symbols 104.
One of important features of an OFDM-based cellular wireless communication system for providing a high-speed wireless data service is to support a scalable bandwidth. A system based on the scalable bandwidth can use various bandwidths including bandwidths of 20/15/10/5/2.5/1.25 megahertz (MHz), etc. A service provider can provide each cell with a service by using a bandwidth selected from among such various bandwidths, and there may be many kinds of User Equipment (UE) from a UE enabling a service with a bandwidth up to 20 MHz to a UE supporting only a minimum bandwidth of 1.25 MHz.
In a scalable bandwidth-based system, a UE initially accessing the system must succeed in a cell search without knowing a system bandwidth. Through the cell search, the UE acquires a cell identifier (ID) and synchronization between a transmitter and a receiver for data and control information demodulation. The system bandwidth may be acquired from a synchronization channel (SCH) in the middle of the cell search, or may be acquired by decoding a broadcast channel (BCH), which is a common control channel for system information transmission, after the cell search.
The BCH is a channel for transmitting system information of a cell that the UE accesses, and the UE initially demodulate the BCH after the cell search is finished. Accordingly, the UE initially performs a cell search through the SCH. After successfully searching for a cell, the UE acquires system information for the cell by receiving the BCH. By reading the BCH, the UE acquires system information necessary for receiving a data channel and other control channels, such as a cell ID, a system bandwidth, channel setup information, etc., cell by cell.
FIG. 2 a conventional example of mapping frequency resources of an SCH 204 and a BCH 206 according to system bandwidths in a system supporting a scalable bandwidth.
In FIG. 2, the abscissa axis 200 represents a frequency, and the SCH 204 and the BCH 206 are transmitted with a bandwidth of 1.25 MHz in the center of a system band irrespective of the system bandwidth. Thus, a UE finds a Radio Frequency (RF) carrier 202 corresponding to the center frequency of the system band irrespective of system bandwidths, and performs a cell search for the SCH 204 in a 1.25 MHz band centered at the RF frequency 202, thereby acquiring initial synchronization for the system. After the cell search, the UE acquires system information by decoding the BCH 206 that is transmitted in the same 1.25 MHz band.
One important problem in a system supporting a scalable bandwidth is to design data and control channels in such a manner that a cell search for an SCH and BCH reception from neighboring cells can be facilitated even when a UE with a reception bandwidth smaller than a system bandwidth is serviced in a partial system band.
Various UEs capable of supporting different bandwidths may exist within a system supporting a scalable bandwidth. As an example, FIG. 3 conventionally illustrates how to allocate UEs 310, 312, 314, 316, 318, 320, each of which is in an active or idle mode and has a reception bandwidth of 10 MHz or 20 MHz, within a system band.
Referring to FIG. 3, when the UEs 310 to 320 accessible to a system have a minimum reception bandwidth of 10 MHz, Multimedia Broadcast Multicast Service (MBMS) physical channels, that is, MSMS#1 300 and MBMS#2 302, are transmitted in respective 10 MHz bands within the 20 MHz system band. The MBMS channels 300 and 302 are channels for providing many users with unidirectional services by using a broadcast or multicast scheme, various broadcast services are provided through the MBMS#1 300 and the MBMS#2 302. Also, an SCH 306 and a BCH 308 are transmitted in a band centered at an RF carrier frequency.
UE#3 314 that is in an idle mode and has a minimum reception bandwidth of 20 MHz can normally receive all of the MBMSs 300, 302, the SCH 306 and the BCH 308. UE#4 316 that is in an idle mode and receives no MBMS service needs to continually receive the SCH 306 and the BCH 308 from neighboring cells and perform a cell search and system information reception while being located in the middle 10 MHz band of the system band, so as to make preparations for a case where the UE#4 316 enters an active mode.
In contrast with this, each of UE#1 310 and UE#2 312 with reception capability corresponding to a bandwidth of 10 MHz receives the MBMS channel 300 or 302 in the upper or lower half band including a desired broadcast channel. However, since UE#1 310 and UE#2 312 are also in an idle mode, they need to receive not only MBMS data but also the SCH 306 and the BCH 308 from neighboring cells, as in the case of UE#4 316, so they can make preparations for entrance into an active mode. Nevertheless, UE#1 310 and UE#2 312 receive only partial bands of the SCH 306 and the BCH 308. It is possible to perform a cell search only by using a sequence of a partial band of the SCH 306, but it is difficult to normally decode system information without receiving all carrier symbols in a band constituting the BCH 308. Similar to this, UE#5 318 and UE#6 320 that are in active mode and located in the upper and lower half bands also have the same problem.
In order to normally decode the BCH 308, UE#1 310 and UE#2 312 must be operative to change their several reception RF frequencies to a band in which the BCH 308 is transmitted (a BCH transmission band), receive the BCH 308, and then return back to the half band in which the MBMSs 300 and 302 are transmitted. In such a case, however, there is a problem in that it may be difficult to receive MBMS data and perform a neighboring cell search without a hitch. Therefore, there is a need for an SCH and a BCH such that UEs can smoothly move between cells without changing their several reception RF frequencies.